This invention relates generally to industrial combustion systems, and more particularly to methods and systems for removing carbon dioxide (CO2) from combustion flue gases.
At least some known carbon separation technologies intervene at different points in coal and/or natural gas systems. For example, carbon separation technologies that separate CO2 from combustion flue gases are generally known as post-combustion carbon separation technologies. Known post-combustion carbon separation technologies include processes such as, but not limited to, physical absorption, cryogenic separation, solid sorbent separation, chemical looping combustion, chemical absorption, and/or membrane separation.
Some known chemical absorption processes attempt to remove CO2 from the flue gases by an exothermic reaction of CO2 with separation solvents, for example, potassium carbonate, sodium hydroxide, and amine-based solvents. Known amine-based liquids may include alkanol amines, for example, diethanolamine, triethanolamine, activated methyl diethanolamine, and monoethanolamines (MEA). During a known chemical absorption process, for example, a flue gas and an amine-based liquid such as MEA counter-currently flow within an absorber (scrubber). The flue gas may enter the scrubber near a bottom end, flow upward, and exit near an opposing top end. The liquid may enter the scrubber near the top end, flow downward, and exit near the bottom end.
A CO2-rich liquid amine-based solution is formed by a chemical reaction of the flue gas and the MEA liquid in the scrubber. The CO2-rich liquid is then channeled to a desorber (stripper). The stripper heats the CO2-rich liquid to reverse the chemical reaction such that the absorbed CO2 is released from the liquid. The released CO2 may be subsequently compressed and transported to storage, and the CO2-lean liquid may be recycled and reused in the scrubber.
The combustion flue gas stream generally includes a smaller volume of CO2 as compared to a larger volume of the flue gas. Known scrubbers generally require equipment sizes capable of processing the larger volumes of flue gas. During processing within known scrubbers, the flue gas is dispersed into the liquid causing gas bubbles to be formed within the liquid. The CO2 absorption amount of the liquid partially depends on a total gas-liquid contact area, which is the sum of the surface areas of the gas bubbles. The liquid may absorb CO2 and other impurities, for example, carbon oxysulfide and carbon bisulfide. Such known impurities may cause foaming of the liquid and/or liquid degradation due to irreversible reactions with the impurities. Also, a driving force that is required to separate the CO2 from the flue gas is determined based on a concentration (density) of flue gas components. The scrubber footprint and stripper regeneration energy increases capital cost, operating costs, and energy consumption. A plant capacity is also reduced because of electrical power consumption in known chemical absorption processes
Some known membrane separation processes include porous membranes that allow selective permeation of gases. The CO2 absorption amount in the liquid partially depends on the physical interaction between flue gases and membrane materials, for example, polyimide and polyolefin. Membrane materials and pore sizes partially affect the degree in which one flue gas component permeates the pores as compared to other flue gas components. Compression of the flue gas is generally used to provide the driving force for permeation because a driving force that is used to separate the CO2 from the flue gas is a pressure differential across the membrane. Therefore, a separation solvent and a stripper are not required for membrane CO2 separation as compared to known chemical absorption processes. Additional compression of the separated CO2 may be used for CO2 transport and/or storage. Although known membrane separation processes generally use smaller scrubber sizes, such known scrubbers may produce a lesser amount of separated CO2 as compared to CO2 released using a chemical absorption process. As such, multiple recycling and processing of the flue gas may be needed in a smaller scrubber to achieve the same degree of CO2 separation as a larger scrubber that processes a similar amount of flue gas. The additional flue gas processing and compression further increase energy consumption and costs.